A LATE CENOZOIC KINEMATIC MODEL FOR FAULT MOTION WITHIN THE GREATER CASCADIA SUBDUCTION SYSTEM (Invited Presentation)

Relatively low fault slip rates have complicated efforts to characterize seismic hazards associated with the diffuse subduction boundary between North America and offshore oceanic plates in the Pacific Northwest region. A kinematic forward model that encompasses a broader region, and incorporates seismologic and geodetic as well as geologic and paleomagnetic constraints, offers a tool for constraining fault rupture chronologies—all within a framework of relative motion between the Juan de Fuca, Pacific, and North American plates during late Cenozoic time.

Our kinematic model tracks motions as a system of rigid microplates, bounded by significant mapped faults or approximations of zones of distributed deformation. The scope of the model extends eastward to the rigid craton in Montana and Wyoming, and southward to the Sierra Nevada block of California to provide important checks on its internal consistency. The model reproduces observed geodetic velocities [McCaffrey et al., 2013, JGR] for 6 Ma to present, with generally slightly faster motion for 12–6 Ma. Constraints for the older deformation history are based on paleomagnetic rotations within the Columbia River Basalt Group, and geologic age dating of fault offsets. Since 17 Ma, our model includes 50 km of N-S shortening along 120°W in the central Yakima fold and thrust belt in south-central Washington, substantial NW-SE right-lateral strike slip distributed among faults in the Washington Cascade Range, ~90 km of shortening on thrusts of Puget Lowland along 123°W, and substantial oroclinal bending of the Crescent Formation basement surrounding the Olympic Peninsula. The large shortening in western Washington suggests that many faults should be considered seismically active.

This quantitative reconstruction provides an integrated framework with which to investigate the motions of various Pacific Northwest forearc and backarc blocks during late Cenozoic time. By explicitly defining major known fault blocks and their Euler poles, the framework offers a preliminary platform for constructing a UCERF3 type model to characterize seismic risk and seismic hazard in the Pacific Northwest region.